1. Field of the Invention
This invention relates to a push button switch primarily used as a keyboard switch, such as in a keyboard for a data input-output terminal unit, and, more particularly, to such a push button switch having an improved plunger stroke converting mechanism for actuating a dome spring switch element, affording an enhanced snap action, and which is of compact size and high reliability and may be produced at low cost.
2. Description of the Prior Art
There are several types of conventional push button switches, for example, switches using a mechanical contact element, switches using a non-contact switch element such as a Hall element, switches using a conductive membrane, and the like. The present invention relates to the first type switch, and employs a dome spring as a mechanical contact element.
In order to achieve good operational characteristics, e.g., both tactile and audible, of a push button switch with respect to a keyboard operator, the push button switch should function to close the switch contacts in response to the application of a key-actuating, or key-depressing, force onto the key top in the range of from 50 to 70 grams, and should provide a snap action. The snap action, more specifically, should afford a sudden decrease of the above-noted depressing force, i.e., a depressing force differential, of more than 15 grams. This depressing force differential is referred to briefly hereafter as a "snap force."
A dome spring is a suitable contactor element for closing and opening a circuit between outer and inner contacts of the switch element of such a push button switch. An example of a push button switch employing a dome spring is disclosed in U.S. Pat. No. 4,370,533, issued to S. Kamei, H. Nabetani, and R. Kinoshita on Jan. 25, 1983.
The fundamental structure of a prior art switch element employing a dome spring is shown in FIGS. 1 and 2. FIG. 1 is a synoptic, schematic and cross-sectional view of a push button switch 1 comprising a switch element 10a and a key assembly 10b. The switch element 10a comprises a dome spring 11 and a terminal plate 12 which is made of molded insulating material and supports therein outer and inner contacts 13 and 14, having respective lead terminals 15 and 16. The dome spring 11 has a generally circular periphery and is received within a recess 24, having a corresponding, generally circular periphery, formed in the terminal plate 12 so as to maintain a normally upwardly convex configuration, as illustrated by the cross-sectional view of FIG. 1. The key assembly 10b comprises a key top 17 having a plunger 18 which is received in telescoping, sliding relationship within a corresponding opening 25 provided in a top portion of a housing 50 (the latter shown only schematically, as a fragmentary segment, in FIG. 1). By downward depression of the key top 17, the plunger 18 moves downwardly through the opening 25 and its motion is transmitted to the dome spring 11 by means of a coil spring 20 and an actuator 21. The actuator 21 is pivotally mounted at its end 22 to the terminal plate 12, and includes a downward protuberance 23 corresponding to the central position of the dome spring 11. In response to the pressing action of the protuberance 23 caused by the movement of the actuator 21, the dome spring deforms, i.e., is invented, from its normal, upwardly convex shape to an upwardly concave (i.e., downwardly convex) shape, in which it closes the circuit between the outer contacts 13 and the inner contacts 14. When the depressing force on the key top 17 is removed, dome spring 11 and coil spring 20 return to their initial states due to their elastic restoring forces, raising the actuator 21 and the associated plunger 18 and key top 17 to their normal, rest positions, and opening the circuit between the outer contacts 13 and the inner contacts 14.
The dome spring characteristics are a function of various known design parameters, such as its diameter, thickness, radius of curvature, stiffness of material, etc. An example of the force-displacement characteristics of a dome spring used as a contactor in a push button switch is shown in FIG. 3. The curve shows that the displacement, plotted along the abscissa, is very small, whereas the required depressing force, plotted along the ordinate, is very large; as a result, the use of a direct drive would not afford a comfortable finger touch, i.e., tactile response, for the operation.
Therefore, the actuator 21 having a lever function and the coil spring 20 are employed (i.e., seen in FIG. 1), for reducing the level of the depressing force which must be applied to the key top 17 to produce adequate displacement thereof. The resultant key top force-displacement characteristics are shown in FIG. 4. The curve shows a snap action characteristic, occurring at "X" on the curve and corresponding to a specified relationship of the applied depressing force and resultant key top displacement; the snap action affords both audible and tactile feedback to the operator, which both make the operator feel comfortable and contribute to avoiding mistakes.
FIG. 2 is another synoptic illustration of a prior art push button switch corresponding closely to that of FIG. 1, but wherein the coil spring 20 and the actuator 21 of FIG. 1 are combined into a single actuator 21' which has elastic characteristics and is deformable. Remaining structures of the switch of FIG. 2 are the same as those in FIG. 1 and corresponding reference numerals identify the same or similar parts.
The push button switches illustrated in and explained with reference to FIGS. 1 and 2 have a problem of requiring a relatively long actuator 21 compared with other components of the switch. This arises since the force required to deform the dome spring 11 sufficiently so as to produce its snap action is of approximately a few hundred grams, the exact amount depending on the specific dome spring design; that force, however, is from two (2) to five (5) times the force of 50 to 70 grams which is considered to be preferable for the operator's finger touch. Therefore, the actuator 21 must afford a lever function and accordingly must have a total length which is several times the length of the segment thereof which extends between the pivotally mounted end 22 and the protuberance 23. Therefore, the prior art, dome spring-type of push button switches require an undesirably large housing to accommodate the long actuator, or, alternatively, the switch-element/actuator assembly and the key-top/plunger assembly must be separately mounted in the actual keyboard construction. Conversely, if the switch is to be assembled in a compact housing, it is difficult to achieve the snap action at the desired, low level depressing force.